As we approach the spring and the peak of storm season, it is time once again to think about lightning protection, both personal protection and for our equipment.
Lightning, What is it?
Lightning is best defined as a massive complex AC surge with a typical frequency of about 20 to 500 Khz. It is not a DC surge as many people think. Lightning usually takes the form of a pulse that has a rise time of about 2 microseconds and a decay time of between 10 to 45 microseconds. If you use the frequency formula of f=1/t you will find that the initial pulse is about 1/.000002 or 500000 hertz (500 Khz) and the decaying pulse is 1/.000045 or 22222.222 hertz (22 Khz). The IEEE "Standard" strike is defined as a 8 us (rise time) by 20 us (fall time) with an average current of 18,000 amps for the first stroke and half that for the second and third strokes. An average strike is three to four strokes. Because we are dealing with an AC waveform, DC resistance to ground is not nearly as important as the INDUCTANCE to ground.
Any bend or coil in the ground system adds inductance. For this reason, any run to ground must be as straight and free of bends as possible. Gentle bends are preferred to right-angle bends. Since we are dealing with a complex AC wave form with rapidly changing frequency, the majority of the currents is carried near the "skin" of the conductor, so the more surface area, the better. Wide copper "straps" work better than thick round solid cables. Multi-stranded cables (the more strands the better, welding cable for example is preferred to AC Cable) work better than solid cables. The green wire AC ground in your house or apartment is USELESS as a ground for lightning protection. It has lots of bends, is coiled in places, and is usually quite long (or resistive), so this presents a huge inductance to the pulse.
Most damage to our homes and equipment is not caused by a direct strike but by huge "induced" voltages on conductors from a nearby strike. If you remember that only an AC waveform can cause induction, we then are dealing with the phenomena of EMP or Electro-Magnetic Pulse.
The Station Ground for Lightning Protection
If there is one rule of grounding for lightning protection, it's USE A SINGLE POINT GROUND SYSTEM. All of the coax, rotor cables, AC wiring, wiring boxes and telephone wiring (if any) should come together and be bonded together at one single point in the equipment area. Having multiple places where these items go to ground leads to a condition known as a ground loop. Each ground point in a multi point ground will have its own resistance and inductance. This leads to different voltage levels and currents seeking to go to ground. Lightning seeking the easiest path to ground will flow around these various ground paths until it finds the best path for itself. Ground loop currents flowing around these other paths are usually the cause of equipment damage, not the lightning strike itself.
Surge suppressors and other treatments
After you have a good ground established, then it's time to investigate surge supression systems. There are several commercial surge suppressors on the market that seem to work well. Transi-Trap, Polyphaser and MFJ offer "in line" coaxial devices. Most of these are based around a "gas filled" spark gap that will protect your equipment. These devices will literally kill themselves to protect your equipment. After a severe strike they may have to be serviced or replaced. Most offer a replacment element for this reason.
Treatments to the telephone and AC wiring in the shack should consist of placing M.O.V devices or transient suppressors across the line. S we will be dealing with induced voltages, a single device is not enough for the AC wiring. A device from Hot to Ground, Hot to Neutral, and Neutral to Ground affords the best protection.
In the Field
Many of us do storm spotting for R.A.C.E.S, so what is safest place while storm spotting? The enclosed car. Because it is isolated from ground by the rubber in the tires, it's usually the safest place. The car is wrapped around your body in what is termed as a Faraday Cage (a sheathed electrical cage isolated from ground). In the event of a direct strike, the car will instantly come up to the same potential as the strike. It's the differences in potential that cause damage and injury.
One of the Ace storm chasers for this area is K5KJ. He often tells the story of a lightning strike to a telephone pole near where he was parked (about 20 feet). Even though the strike damaged the majority of the radio equipment in the car, he managed to get the car started and drive away without injury.
If you are out of the car, avoid tall objects such as trees, power and telephone poles, and stand with your feet close together. This avoids having a difference in potential of several thousand volts between your feet and your heart.
The best way to avoid injury and damage would, of course, be to disconnect the equipment and not use it during a storm. However in our role as emergency communicators, we are often called upon to communicate in less than ideal conditions. Please remember that equipment is replaceable, but YOU are not. Take all possible precautions when communicating for R.A.C.E.S or Civil Defense activities this spring.
Safe Storm spotting and Good luck,
Copyright 2005 Gerald Crenshaw WD4BIS. All rights are reserved.
Permission in advance is granted to those who use this for non-profit Amateur Radio club newsletters as long as it is used unmodified including this copyright notice and that notice is given to the author via email ([email protected]). In addition, please forward a copy of any newsletter this appears in to: Gerry Crenshaw WD4BIS, c/o GARC, 1027B W. Austin St, Garland, TX, 75040
Web site maintained by Janet Gobeille Crenshaw (WB9ZPH)